Sync Chapter: Add explanation on why the concurrent test fails

[laerson]

Problem

A common pitfall for people learning about concurrency is why expressions like x = x + 1 can give unexpected results when it happens on multiple threads, since they look like a single instruction.

On the Sync chapter, the implementation of Inc() method isn't concurrently safe, but the reason why can be confusing for someone with no background on assembly:

func (c *Counter) Inc() {
	c.value++
}

Proposed Solution

Highlight that although c.value++ looks like a single instruction, the compiled instruction is actually similar to this:

tmp := x + 1 // instruction A
x = tmp      // instruction B

And then show how the counter can fail depending on the order the goroutines are executed.